1. Field of the Invention
This invention relates to a low isoflavones, high saponins vegetable material and a process for producing the same.
2. Description of the Related Art
The benefits of soy protein are well documented. Cholesterol is a major concern with consumers throughout the industrialized world. It is well known that vegetable products contain no cholesterol. For decades, nutritional studies have indicated that the inclusion of soy protein in the diet actually reduces serum cholesterol levels in people who are at risk. The higher the cholesterol, the more effective soy proteins are in lowering that level.
Soybeans have the highest protein content of all cereals and legumes. In particular, soybeans have about 40.0 wt. % protein, while other legumes have 20.0–30.0 wt. %, and cereals have about 8.0–15.0 wt. % protein. Soybeans also contain about 20.0 wt. % oil with the remaining dry matter mostly carbohydrate (35.0 wt. %). On a wet basis (as is), soybeans contain about 35.0 wt. % protein, about 17.0 wt. % oil, about 31.0 wt. % carbohydrates, and about 4.4 wt. % ash.
In the soybean, both protein and lipid bodies are contained in the usable meat of the soybean (called the cotyledon). The complex carbohydrate (or dietary fiber) is also contained in the cell walls of the cotyledon. The outer layer of cells (called the seed coat) makes up about 8.0 wt. % of the soybean's total weight. The raw, dehulled soybean is, depending on the variety, approximately 18.0 wt. % oil, 15.0 wt. % soluble carbohydrates, 15.0 wt. % insoluble carbohydrates, 14.0 wt. % moisture and ash, and 38.0 wt. % protein.
In processing, soybeans are carefully selected for color and size. The soybeans are then cleaned, conditioned (to make removal of the hull easier) and cracked, dehulled and rolled into flakes. The flakes are subjected to a solvent bath that removes the oil. The solvent is removed and the flakes are dried, creating the defatted soy flakes that are the basis of all soy protein products. Despite the large number of products on the market, there are only three types of defatted soy protein products: flours, concentrates, and isolates.
Soy flours are the simplest forms of soy protein, having a protein content of approximately 50.0 wt. %. Simply grinding and screening the defatted flakes produces soy flours. This simple processing leaves the soy flour with many of the soybean's characteristics. The lack of processing also makes soy flours highly variable in terms of quality.
Soy flours and grits are still widely produced and are used most often in baked goods, snack foods and pet foods applications where the high flavor profile does not pose a problem. Textured soy flours were an early attempt at simulating or enhancing the texture of meat products. Texturizing does not change the composition of soy flours and reduces the flavor profile only slightly. Their primary applications are inexpensive meat products or pet foods.
Soy concentrates have at least 65.0 wt. % protein on a moisture-free basis. Soy protein concentrates are made by removing soluble carbohydrate material from defatted soy meal. Aqueous alcohol extraction (60–80% ethanol) or acid leaching (isoelectric precipitation) are the most common means for carbohydrate removal. In both aqueous alcohol extraction and acid leaching, however, essentially all of the protein is rendered insoluble. Protein solubility may be partially recovered in acid leach products by neutralization. Also, in aqueous alcohol extraction, the isoflavones and saponins are extracted into the alcohol phase, and are subsequently discarded. Thus, soy protein concentrates which are produced using aqueous alcohol extraction have at most only trace amounts of isoflavones and saponins. A myriad of applications have been developed for soy concentrates and texturized concentrates in processed foods, meat, poultry, fish, cereal and dairy systems.
Isolates are produced through standard chemical isolation, drawing the protein out of the defatted flake through solubilization (alkali extraction at pH 7–10) and separation followed by isoelectric precipitation. As a result, isolates are at least 90.0 wt. % protein on a moisture-free basis. They are sometimes high in sodium and minerals (ash content), a property that can limit their application. Their major applications have been in dairy substitution, as in infant formulas and milk replacers.
Isoflavones occur in a variety of leguminous plants and oilseeds, including vegetable protein materials such as soybeans. These compounds generally include daidzin, 6″-O-acetyldaidzin, 6″-O-malonyldaidzin, daidzein, genistin, 6″-O-acetylgenistin, 6″-O-malonylgenistin, genistein, glycitin, 6″-O-malonylglycitin, glycitein, biochanin A, and formononetin.
It has recently been suggested that the isoflavones contained in vegetable proteins such as soybeans may inhibit the growth of human cancer cells, such as breast cancer cells, prostate cancer cells and colon cancer cells. In addition, isoflavones also have been suggested to reduce cardiovascular risk factors, for example by reducing the levels of atherosclerosis inducing lipoproteins and low-density cholesterol and by increasing endothelial dependent vasodilation response. Isoflavones are also showing great promise in preventing osteoporosis and treating menopausal symptoms.
Isoflavone compounds have been associated with an inherent, bitter flavor in vegetable protein materials such as soybeans. In the commercial production of such protein materials, such as protein isolates and protein concentrates, the focus has been to remove isoflavone compounds. For example, in a conventional process for the production of a soy protein isolate, soy flakes are extracted with an aqueous medium having a pH above the isoelectric point of the protein to solubilize the protein. The extract containing the protein is separated from insoluble fiber materials to provide a protein extract. Most of the isoflavones are solubilized in the extract as well as the protein. The protein is precipitated by acid leaching, i.e., adjusting the pH of the extract to about the isoelectric point of the protein, typically between 4.2 and 4.6 for soy protein, with an acid. The precipitated protein is then separated from the extract. Much of the isoflavones remain solubilized in the extract following separation of the precipitated protein (curd) from the extract; however, some of the isoflavones are usually present in the precipitated curd. After separation of the precipitated protein curd from the extract, the extract and the isoflavones solubilized therein are usually discarded. Any residual isoflavones left in the separated protein are removed by exhaustive washing of the protein to ensure that the taste associated with the isoflavones is not present in the protein. Also, the foregoing washing process tends to remove saponins as well as isoflavones.
Problematically, isoelectric precipitation reduces the solubility of the proteins. Therefore, soy protein concentrates and isolates made using the foregoing process usually have low solubility, as indicated by their low Nitrogen Solubility Index (“NSI”), which is typically less than about 70%.
Soybeans contain about 0.5 wt.% saponins. Soy saponins have been the subject of investigation since the early 20th century. These compounds consist of a triterpenoid skeleton with various sugar and acetyl moieties. The current consensus is that soyasapogenols A, B, and E are true aglycons, while soyasapogenols C and D are artifacts of hydrolysis that occurs during the process of their isolation. The corresponding glycosides are the so-called “group A saponins,” “group B saponins,” and “group E saponins,” respectively.
Soy saponins have been suggested to demonstrate anti-mutagenic properties that make them promising agents for cancer prophylaxis. Moreover, it has been suggested that group B soy saponins have exhibited pronounced suppressive effects on the replication in vitro of the human immunodeficiency virus (HIV). The chemical structure of soybean saponins is very similar to that of the compound glycyrrhizin, a known anti-viral agent, so soy saponins show promise as building blocks for the synthesis of anti-viral pharmaceutical compounds.
Despite the cultivation and processing of very large quantities of soybeans, at the present time soy saponins are not a significant article of commerce due to the difficulty of isolating and purifying them.